Separation of liquids



y 1961 w. NOCK ETAL' SEPARATION OF LIQUIDS 2 Sheets-Sheet 1 Filed July 22, 1957 y B W M a M y I'TOR/VEVS May 23, 1961 w. NOCK ETAL SEPARATION OF LIQUIDS Filed July 22, 1957 2 Sheets-Sheet 2 EJ ea e4 [D M m .E H m n m E M W 1 PM Y B mam SEPARATION'OF LIQUIDS William Neck and Thomas Robert Prentice, Saltcoats,

and Roy Bentley, Largs, Scotland, assignors to Imperial Chemicallndustries Limited, Millhank, London, S.W. 1, England, a corporation of Great Britain Filed July 22, 1957, Ser. No. 673,387 Claims priority, application Great Britain Aug. 13, 1956 Claims. (Cl. 210-83) This invention relates to manufacturing processes which include the step of separating at least two liquids of different density and different electrical conductivity and more particularly in a continuous process for the manufacture of a liquid explosivenitric ester to the step of separating said ester from the spent nitrating acid mixture. In such a process a separator for instance may be continuously fed with an emulsion of nitroglycerine in a mixture of nitric acid and sulphuric acid and as the nitroglycerine separates and rises and the mixed acid sinks three layers are formed, namely one of nitroglycerine, one of mixed nitric and sulphuric acids, and one of an emulsion of nitroglycerine in the mixed acids sandwiched between said layers. It is usual continuously to admit the said emulsion from a nitrator to the separator at a point intermediate between the top and the bottom and continuously to withdraw nitroglycerine from near the top of the separator and refuse acid from near the bottom. For efliciency of separation 'it is desirable to keep the interface between the nitroglycerine and the emulsion and the interface between the emulsion and the spent acid between predetermined limits vent the nitroglycerine emulsion from entering the refuse acid line. if the top interface rises or falls beyond a predetermined limit it is usual to make the necessary correction by varying the rate of withdrawal of the refuse acid and if the lower interface falls below a predetermined limit it is desirable to stop the production of th nitroglycerine emulsion.

The object of the present invention is to provide a process which permits the position of an interface between adjacent liquid layers which are formed when a mixture of liquids of different density and different electrical conductivity is separated in a separator to be kept within a stipulated range from a predetermined position more conveniently than heretofore and to provide apparatus therefor to permit said process to be carried out a distance from the separator either automatically or by visual or aural indication for manual operation. According to the present invention the method which permits keeping within a stipulated range from a given position an interface between adjacent liquid layers which are formed when a mixture of liquids of different density and different electrical conductivity is separated in a separator comprises applying a difference of potential to the liquid at a predetermined position, detecting any changes in electrical conductivity of the liquid at said predetermined position, and arranging that the separation is carried out so that the electrical conductivity at said predetermined position is kept within predetermined values.

The method of the invention is particularly applicable United States Patent O "ice 2,985,305 Patented May 23, 19ti1 2 to a mixture of two liquids of different density and different electrical conductivity as for example an emulsion 'of nitroglycerine and spent nitrating acid.

An interface between adjacent liquid layers can be kept between a stipulated range from a given position 'for example simply by applying a difference of potential On the other hand an interface between adjacent liquid layers can be kept between a stipulated range from a given position by applying differences of potential to the liquid at at least two predetermined positions, detecting any changes in electrical conductivity of the liquid at said two predetermined positions, and arranging that the separation is carried out so that the electrical conductivity at each of said two predetermined positions is kept within predetermined values.

For an emulsion of two liquids, e.g. an emulsion of nitroglycerine and spent nitrating acid, which gives in a separator three liquid layers namely a layer of the emulsion and a layer for each of the two liquids wherein for instance there is an interface between the layer of the less dense liquid and the layer of emulsion and an interface between the layer of emulsion and the layer of the liquid of higher density thenit may be desirable to have a given position for each interface and to keep each interface between a stipulated range from its particular given position when it is desirable to apply differences of potential to the liquid at at least three predetermined positions, detecting any changes in electrical conductivity of the liquid at said three predetermined positions, and arranging that the separation is'carried out so that the electrical conductivities at said three predetermined positions are kept within predetermined values.

If said separator is a static separator and if said two liquids are nitroglycerine and a spent nitrating acid in the form of an emulsion then said difference of potential is preferably a difference of alternating potential of a frequency preferably of at least 2 kc./s. and of such voltage that the electrical current is preferably less than 1 ma. In the separation of an emulsion of nitroglycerine and mixed acids in a stainless steel separator it is also desirable that said difference of alternating potential should be applied to stainless steel electrodes and with such electrodes it has been found that alternating currents of 130 ma. can be passed through an agitated mixture of nitroglycerine and a mixture of nitric and sulphuric acids for half an hour without producing any significant change in the stability of the nitroglycerine.

Preferably if the separator is of electrically conducting material and is a static separator it can conveniently -'be one of the electrodes.

If desired, a pair of electrodes can be situated in more than one predetermined position. If the separator consists of electrically conducting material then it can be one of the electrodes of each pair of electrodes and each of a the other electrodes required to form the pairs of elecposed stainless steel rings bonded thereto with their leading wires embedded axially in the polythene.

Any change in conductivity between any particular pair or electrodes to which a difference of potential is applied as will for example be caused by the movement of an interface past at least one of the electrodes forming said pair may be detected in any known manner as for instance by an electrical bridge network containing a voltage detector working in association with an indicator or control unit. The voltage detector may be of any conventional kind or preferably may be a phase sensitive detector comprising a gas-filled tetrode valve which has an alternating voltage of the same frequency as and preferably derived from the electrical source supplying the pair of electrodes applied to its screen grid, the control grid being supplied by an amplified output voltage from the bridge network. The bride network can be so adjusted that when an interface passes the pair of electrodes such a change is caused in the current flowing in the bridge network that the alternating control grid voltage is brought into such :a phase relationship with the alternating screen grid voltage that the gas-filled tetrode valve is made conducting.

The method of the invention can be applied to any form of separator for separating a mixture of liquids'into'liquid layers as for instance a static separator or a centrifugal separator.

The invention will now be described by way of example 'with reference to the diagrammatic drawings attached hereto wherein Figure 1 is an elevation of a continuous earthed stainless steel separator of the kind used in the manufacture of nitroglycerine provided with a set of seven single electrodes and Figure 2 is an electrical diagram illustrating in some detail one of seven electrical bridge networks and its detector and a set of signal lamps six of which are each operated by its detector and two of which are operated by the remaining seventh detector.

' In Figure l, 1 is an earthed stainless steel continuous separator having a glass top portion 2, 3 is a tangentially positioned delivery pipe, 4 is a sight glass, 5 is a valve, 6 is a motor which controls the valve 5, 7 and 8 are relays which are controlled from a distance to operate the motor 6 and 9 is an overflow pipe. 10 is a layer of an emulsion of nitroglycerine and spent nitrating acid, 11 is a layer of nitroglycerine and 12 is a layer of spent nitrating acid. 13 is the top interface and 14 is the bottom interface. 15 is a set of seven single stainless steel electrodes 16, 17, 18, 19, 20, 21, 22 with their respective leading wires 23, 24, 25, 26, 27, 28, 29 which are connected to their respective screened cables 30, 31, 32, 33, 34, 35, 36.

The set 15 of the seven single stainless steel electrodes is shown so immersed in the three layers 10, 11 and 12 that the single electrodes 19 and 20 are on either side of the given position for the top interface 13. If the top 7 interface 13 rises or falls beyond the position of the electrodes 19 and 20 the necessary correction is made by varying the rate of withdrawal ofthe spent nitrating acid 12 through valve 5. If valve 5 has not been opened sufiicient- 1y when'the top interface 13 is rising this will be indicated by indicating means for electrode 18 and if the top interface 13 falls too rapidly this will be indicated by indicating means for electrode 21 when it becomes necessary to adjust valve 5 to diminish the rate of withdrawal of the 2 spent acid 12.

When it is desired to remove all of the nitroglycerine layer 11 from'the separator 1 the valve 5 is closed while the emulsion of nitroglycerine and spent nitrating acid is introduced through pipe 3. The top interface 13 begins to rise and this is indicated in due course by indicating meansfor the electrode 17 when the delivery ofsaid emulsion is discontinued. The top interface 13 rises gradually .and when the indicating means for electrode 16 indicates thatitheilayer ofemulsion 10 has reached the given posi- .-.tion:fr.electrode 16 valve isimmediately opened so that 4 no emulsion is permitted to flow into the overflow pipe 9.

During the continuous separation it is highlydesirable to keep the level of the lower interface 14 above the given position for the electrode 22 and if the lower interface 14 falls below the given position for the electrode 22 as indicated by its indicating means it is desirable to stop the production of the nitroglycerine emulsion.

In Figure 2, 30., 31, 32., 33, 34, 35 and 36 are the extensions of the screened cables shown in Figure 1. Each of the seven electrical bridge networks is made up of resistors 37 and 38, a variable resistor 39 and a capacitor 40. For the first bridge network shown in some detail an alternating voltage at a frequency of 5 kc./ sec. is applied at terminals 41 and 42 'to supply this bridge network through an isolating screened transformer 43 and limiting resistors 44 and 45. The output voltage from this bridge network is fed through the screened cable 46 to the amplifying valve 47 and after amplification to the control grid 48 of the gas-filled tetrode 49. An alternating voltage at a frequency of 5 kc./sec. is also applied to terminal 50 and thereby to the screen grid ,51 in the gas-filled tetrode -49. 52 is a terminal to which a source of direct voltage is applied for energising the amplifying valve 47. '53 is a terminal to which a source of alternating voltage at a frequency of 50 cycles/ sec. is applied to energise the gas-filled tetrode 49. 54 is a terminal to which a negative bias voltage is applied.

When the .top interface 13 passes the single electrode 16 such a change is caused in the current flowing in the bridge network that the alternating voltage on the control J-grid 48 is brought into such a phase relationship with the alternating voltage on the screen grid 51 that the gas-filled tetrode valve 49 is made conducting so that the relay 55 is energised and closes contact '56 to light the signal lamp '57. a

Similarly when the top interface 13 passes the electrode 17, relay 58 associated with the next bridge network, connected and to which an alternating voltage is applied as in the first bridge network, is energised to close contact 59 and to light signal lamp 60. Also when top interface 13 passes the electrode 18 relay 61 associated with the third bridge network, connected and to which an alternating voltage is applied as in the first bridge network, is energised to close contact 62 and to light signal lamp 63. Again when the top interface 13 passes the electrode ,19 relay 64 associated with the fourth bridge network, connected and to which an alternating voltage is applied as 'in the first bridge network, is energised to close contact 65 and .to light signal lamp 66. Also when the top interface 13 passes the electrode 20 associated with the fifth bridge network, connected and to which an alternating voltage is applied as in the first bridge network, 18:611- ergised to close contact 68 and to light signal lamp 69. When the top interface 13 passes the electrode 21 relay 70 associated with the sixth bridge network, connected and to which an alternating voltage is applied as in the first bridge network, is energised to close contact 71 and to light signal lamp 72. Thus the position of the top interface 13 is known to be above the position of the electrode corresponding to the topmost lit signal lamp.

When the lower interface 14 is above the electrode 22 relay 73 associated with the seventh bridge network, connected and to which an alternating voltage is applied as in the first bridge network, is unenergised and lamp 74' is lit through contact 75. When the lower interface 14 is below the electrode 22 the relay 73 is energised and changes over the contact 75 to light signal lamp 76.

Thus, for instance, when the signal lamp 66 is lit indicating that the top interface 13 is above the given position for the electrode 19 valve 5 may be opened by an operator working by remote control the relays 7 and 8 to keep said top interface 13 within its stipulated range.

What we claim is:

1. A method which comprises applying at a predetermined position in a separator to the liquid therein, which includes a liquid explosive nitric ester/nitrating acid mixture, an alternating potential difference such as to pass through said liquid in its emulsion condition an alternating current not exceeding 130 milliamps at a frequency of at least two kilocycles per second, detecting any changes in electrical conductivity of the liquid at said predetermined position, and causing in response to the detection of such changes separation of the mixture to be carried out so that the electrical conductivity of the liquid at said predetermined position is kept within predetermined values.

2. A method as claimed in claim 1 wherein said mixture is an emulsion of nitroglycerine and spent nitrating acid.

3. A method as claimed in claim 1 wherein said separation causes two adjacent liquid layers of differing densities and the resulting interface between said two layers is kept within a stipulated range which includes said position by causing the rate of withdrawal of the liquid layer of higher density and the rate of entry of the mixture to be such that the conductivity at the predetermined position fluctuates at a constant frequency from the conductivity of one of the liquid layers to the conductivity of the other liquid layer.

4. A method as in claim 1 wherein an interface between adjacent diiferent density liquid layers resulting from said separation is kept within a stipulated range which includes said position by applying a second alternating potential difierence, a frequency and magnitude similar to that of the aforementioned potential difference, to the liquid at at least one other and different predetermined position, detecting any changes in electrical conductivity of the liquid at said other predetermined position, and causing in response to the detection of such changes at both said positions separation of the mixture to be carried out so that the electrical conductivity at each of said two predetermined positions is kept within predetermined values.

5. A method as in claim 2 wherein the emulsion gives as a result of said separation three liquid layers namely a layer of the emulsion and a layer for each of the two liquids so that there is an interface between the lay r of the less dense liquid and the layer of emulsion and an interface between the layer of emulsion and the layer of the liquid of higher density and it is desired to have a given position for each interface and to keep each interface within a stipulated range from its particular given position, applying alternating potential differences, each substantially the same in frequency and magnitude as the aforesaid potential difference, to the liquid at at least three predetermined positions, detecting any changes in electrical conductivity of the liquid at said three predetermined positions, and carrying out the separation so that the electrical conductivities at said three predetermined positions are kept within predetermined values.

6. A method as in claim 1 wherein the potential is applied with such voltage that the said electrical current is not more than 1 ma.

7. In the separation of liquid explosive nitric est r/nitrating acid mixtures, the method of maintaining the level of the interface between adjacent liquid layers within a selected vertical positional range in a separator in which there is a series of paired electrodes disposed within the separator in spaced vertical relationship, each pair of said electrodes being in circuit with a source of electrical energy and completing its circuit through liquid in the separator, there being a different indicating means associated with each such electrode pair circuit, comprising the steps of applying an alternating electrical potential having a frequency of at least two kilocycles per second to each of said paired electrodes whereby variations of the liquid level interface between different paired electrodes will be denoted by variations in electrical conductivity, said potentials having a magnitude such as to pass an alternating current of not more than milliamperes through said liquid in its emulsion condition, denoting the level of the said interface by indicating means associated with the electrode circuits and responsive to variations in electrical conductivity due to movement of the liquid interface within the separator, and adjusting the level of the interface by controlling the flow of liquid to and from the separator in accordance with the responses of the aforesaid indicating means.

8. A method as in claim 7 wherein the alternating potentials are applied so that the alternating current through the liquid in its emulsion condition does not exceed 1 milliampere.

9. In the separation of liquid explosive nitric ester/nitrating acid mixtures, in which the liquid explosive nitric ester is nitroglycerine, the method of maintaining the level of the interface between adjacent liquid layers within a selected vertical positional range in a separator in which there is a series of paired electrodes disposed within the separator in spaced vertical relationship, each pair of said electrodes being in circuit with a source of electrical energy and completing its circuit through liquid in the separator, there being a different indicating means associated with each such electrode pair circuit, comprising the steps of applying an alternating electrical potential having a frequency of at least two kilocycles per second to said paired electrodes whereby variations of the liquid interface between dilferent paired electrodes will be denoted by variations in electrical conductivity and to cause an alternating current through said liquid in its emulsion condition of not more than 130 milliamperes, denoting the level of the said interface by indicating means associated with the electrode circuits and responsive to variations in electrical conductivity due to movement of the liquid interface within the separator, and adjusting the level of the interface by controlling the flow of liquid to and from the separator in accordance with the responses of the aforesaid indicating means.

1 0. A method as in claim 9 wherein the said potential is applied so as to cause a current not exceeding one milliampere through the liquid in its emulsion condition.

References Cited in the file of this patent UNITED STATES PATENTS 1,503,956 Klinger Aug. 5, 1924 1,577,981 Otto Mar. 23, 1926 2,140,897 Challenor Dec. 20, 1938 2,523,253 Smith Sept. 19, 1950 2,831,579 Gehle Apr. 22, 1958 

